Sputtering apparatus



June 23, 1970 E. c. MULY ,'JR ET SPUTTERING APPARATUS Filed June 9, 1967 2x10 Torr FIG. I

United States Patent 3,516,920 SPUTTERING APPARATUS Emil C. Muly, Jr., Needham, and Robert M. Oman,

Wellesley, Mass., assignors to National Research Corporation, Newton Highlands, Mass., a corporation of Massachusetts Filed June 9, 1967, Ser. No. 644,923 Int. Cl. C23c 15/00 US. Cl. 204-298 3 Claims ABSTRACT OF THE DISCLOSURE Dielectric targets are sputtered by neutralizing positive ion collection at the target using a separate source of electron bombardment instead of the conventional radio frequency voltage source connected to the target. The apparatus can also be utilized for sputtering conductive targets with the usual high voltage connector going to the electron source instead of the target.

The present invention relates to cathode sputtering apparatus of the type described for instance in US. Pat. 3,021,271 to Wehner. It is a special problem in this art to sputter dielectric material targets because the sputter target must be biased negatively to attract positive ions, but a dielectric cannot drain positive charges formed by impinging ions. The art has solved this problem by utilizing high frequency target voltage sources (Wehner, Advances in Electronics and Electron Physics, vol. 7 (1955)).

The high frequency voltage source adds undue complexity, cost and bulk to the sputtering apparatus and involves problems of tuning and shielding to prevent unauthorized radio broadcasting from the voltage source.

It is the object of the present invention to provide an alternative apparatus for sputtering dielectrics which avoids or is less vulnerable to the foregoing drawbacks.

This object is achieved by using a separate high energy electron gun to charge the electrically floating target. When the target is a dielectric (or an electric floating conductor) the electrons impinging on the target face cause positive ion collection by charging the target negative with respect to the plasma. The single apparatus is useful for sputtering of both conductors and dielectrics. The electron gun is constructed to impinge electrons on a wide area of the target to assure substantial uniformity of charge. The electron beam formed by practical ineX- pensive apparatus is not uniform, per se. But the mobility of electrons reaching the target surface is sufficient to compensate for this deficiency. This apparatus avoids the need for a high frequency power supply with its attendant problems and indeed, can utilize the direct current high voltage power supplies now used for direct connection to conductive targets. The power supply would be connected to the electron gun of the present invention.

Embodiments of the invention are now described with reference to the accompanying drawings wherein FIG. 1 is a diagram of a first and preferred embodiment and FIG. 2 is a similar diagram of a second embodiment.

The apparatus of FIG. 1 comprises a source of electrons 10 which is a thermionic filament 12 inside a grounded metal tube 14 with an exit port 16 adjacent to which is an auxiliary anode electrode 18 and opposed thereto being a main anode electrode 20. There is an electron path between the electron source and anode indicated by line 22. A plasma forms in this region. A target 24 of dielectric material is mounted parallel to the electron path. Opposing the target is a substrate 26 ice to be coated with sputtered target material and having an aperture 28 therein.

Mounted behind the substrate is an electron gun 30 of well known design, per se, after Pierce. The gun comprises the usual elements; heater 32, cathode 34, grid 36 and anode 38. The gun is powered by a conventional industrial power supply 40 with current control 42. In operation the gun produces a wide beam 44 of electrons striking the target to charge it to its desired negative operating potential and neutralize positive ions on the target surface.

The apparaus described above is mounted in a vacuum chamber (not shown) which is evacuated to a pressure of about 2 1O torr as indicated on the drawing. At

these pressures the gun 30 can operate for reasonably long periods of time before operation is limited by erosion of the cathode 34 or arcing due to backstreaming ions generated in the electron beam 44 and/or in the basic plasma formed at 22 between electron source 10 and anode 20.

The sputtering operation can be conducted in the medium provided by residual air molecules or the apparatus evacuated to a lower pressure and backfilled to a pressure of 2 10" torr by bleeding in a desired gas such as argon or hydrogen.

Electrical voltage sources (not shown) are provided to bias the gun cathode to about 4000 volts negative and the anodes 18, 20 to low positive voltages as indicated on the drawing. The magnetic field H is typically about 10-20 gauss for preventing divergence of the low energy electron beam obtained from source 10. This low field has little effect on the high energy electron beam 44 or on the ions formed in the plasma.

A second embodiment of the invention is shown in FIG. 2 wherein the gun 30 is mounted at a right angle to its original FIG. 1 position and the magnetic field strength H is increased to several hundred gauss to bend the electron beam 44 around to target 24. This avoids the use of an aperture in substrate 28.

Other variations within the scope of the present invention will be apparent to those skilled in the art. For instance, the sputtering can be conducted at higher pressures, say 10 torr while a separate pump is provided to evacuate gun 30 to the lower pressure (below 10 torr) necessary for its effective operation. The apertures 39 of anode 38 and 1 or 28 of substrate 26 can be utilized to maintain this pressure differential. The electron gun can be provided with beam bending and other ion trap means as shown in US. Pat. 3,046,936 to Simons. The electron source 30 may be operated in a pulsating fashion to pcriodically charge target 24 if desired. It is therefore intended that the above disclosure shall be read as illustrative and not in a limiting sense.

What is claimed is:

1. In a sputtering device comprising means for forming a plasma and a target located to draw positive ions from said plasma when the target is sufficiently negatively charged for sputtering, the improvement wherein a source of electrons is provided apart from said target and plasma forming means, said source being constructed and arranged to spray electrons on said target to provide said negative charge, at least in part for sputtering.

2. The apparatus of claim 1 wherein the said sputtering device has a substrate in opposing relationship to the target and wherein the said source of electrons is located behind the substrate and wherein the substrate has an aperture, the apparatus comprising means for focussing the electrons through the aperture and spreading them beyond the aperture to cover a large target area.

3. The apparatus of claim 1 wherein the target floats 4 electrically and the said electron source is the principal means for biasing the target. JOHN H. MACK, Primary Examiner References Cited S. S. KANTER, Assistant Examiner UNITED STATES PATENTS 5 U.S. C1.X.R. 3,324,019 6/1967 Laegreid et a1 204298 204 .192 3,287,243 11/1966 Libenza 204192 3,271,286 9/1966 Lepselter 204192 

